1. Field of the Invention
The present invention relates to a semiconductor laser device, and more specifically, it relates to a semiconductor laser device having a structure of a composite device having a semiconductor laser and an electric device for modulating the optical output thereof made integral with each other.
2. Description of the Prior Art
Recently, a semiconductor laser has come into practical use such as a device for reading signals of optical disks, for optical communication, or for a light source for writing into an optical disk. In the case where a semiconductor laser is used for optical communication or for writing into an optical disk, the optical output of the semiconductor laser needs to be modulated by some means or other. So far, the optical output of a semiconductor laser has been modulated by changing or turning on and off the current flowing through the semiconductor laser according to a prescribed signal by means of a driving/modulating circuit comprised of a transistor, IC, or the like, provided separately from the semiconductor laser.
As an example of such conventional semiconductor laser devices, a device having a semiconductor laser and a field effect transistor for modulation integrated in a horizontal direction on a substrate is disclosed in T. Fukuzawa, et al., Applied physics Letters 36 (3), 1 February 1980, p. 181, "Monolithic Integration of a GaAlAs Injection Laser with a Shottky-Gate Field Effect Transistor".
However, the conventional method for modulating optical outputs of a semiconductor laser by means of a driving/modulating circuit provided separately from the semiconductor laser has drawbacks such as high cost due to the complicated circuit structure, or low system reliability due to the increase of the number of parts. Recently, since most of the signal processing ICs or LSIs are formed with MOS transistors, these ICs and LSIs generally can drive small currents only. Consequently, in the case of a semiconductor laser which requires some 10 mA of a driving current, a bipolar IC or a transistor having a large driving current need be additionally provided causing another problem that the cost of the driving/modulating circuit increases.